1. Field of the Invention
The present invention relates to a total optical conversion circuit for converting NRZ signals to an RZ or soliton format. The present invention also relates to an optical transmission line suitable for transmitting the RZ or soliton signals thus converted. Specifically, this optical transmission line is an optical fiber transmission line in which signals can be transmitted over long distances by selecting an appropriate setting for the dispersion of the transmission lines constituting this optical fiber transmission line.
2. Description of Related Art
It is believed that total optical signal processing circuits will become indispensable for optical communications systems featuring optical fiber transmission lines. In the total optical signal processing circuits currently in the research stage, RZ signals are used as the optical signal format because of good correlation with optical time-division multiplexing. Total optical NRZ/RZ conversion technology is believed to be necessary for future ultra high-speed signal processing.
The following is a description of the general structure of a so-called transponder, which converts an NRZ signal to an RZ signal. As indicated, for example, by Yoneyama et al. in xe2x80x9cAll-optical clock recovery from NRZ data signal using Mach-Zehnder interferometer with different path lengthxe2x80x9d: 1998 Denshi Johotushin gakkai sogo taikai, Transmission 2, B-10-145 (Literature 1), an NRZ signal is inputted to a delay Mach-Zehnder interferometer and is split in two. The two halves travel along optical paths having mutually different lengths, and are then synthesized again. In the process, an optical signal representing the synthetic sum and an optical signal representing the difference resulting from the two halves canceling each other are outputted from two different output ports of the delay Mach-Zehnder interferometer. Of these signals, the optical signal representing the difference contains the clock component of the inputted optical signal, so this signal is inputted to a mode-locked laser diode whose locking range has the same frequency band, and the RZ clock component is extracted by subjecting this laser to optical injection locking.
In this case, however, only the RZ clock component of the optical signal is extracted, and there is no reproduction of the digital pattern contained in the NRZ optical signal. In addition, a mode-locked laser diode (MLLD) is used in Literature 1 above, but the repetition frequency (frequency locking range) of an MLLD is substantially determined by the resonator length of the laser. With current MLLDs, the desired resonator length is obtained by the cleavage of semiconductor chips, so a mode-locked laser diode suitable for the operating frequency of the system is more difficult to obtain than a regular laser element.
The following is a description of an optical fiber transmission line for transmitting an RZ signal or soliton signal thus generated.
Optical transmission lines used in conventional practice are obtained by connecting, for example, transmission fibers to dispersion-compensated fibers or other components whose wavelength dispersion is opposite in sign to the wavelength dispersion of the transmission fibers. For example, the method disclosed in xe2x80x9c4xc3x9720 Gbit/s soliton WDM transmission over 2000 km With 100 km dispersion-compensated spans of standard fiberxe2x80x9d: Electronics Letters Vol. 33, No. 14, pp. 1234-1235, Jul. 3, 1997 (Literature 2) is known as a method for stabilizing so-called dispersion-managed solitons propagating along such optical transmission lines. Literature 2 discloses so-called pre-chirp, a technique in which a short optical pulse is provided with a chirp before this pulse is introduced into a transmission line.
In systems such as those described in Literature 2 above, however, optical pulses that are constantly up-chirped in linear fashion propagate toward the outlet of the transmission line. The width of the stable optical pulses at the outlet of the transmission line is therefore greater than the width of freshly formed pulses. Wide optical pulses are unsuitable for obtaining higher bit rates in optical transmission systems. In addition, the necessary amount of pre-chirp in the pre-chirp technique is determined by the properties of the dispersion-compensated fiber. Applying this method to optical wavelength-division multiplexing transmission causes the chirp amount to vary with the wavelength and makes it difficult to design a transmission line when the dispersion-compensated fiber has a dispersion slope.
A need therefore exists for a simply structured and highly reliable NRZ/RZ converter for converting optical NRZ signals to RZ signals or soliton signals. A need also exists for an optical transmission line through which optical pulses thus generated could propagate in a stable manner.
According to a first aspect of the present invention, an optical signal generation circuit is provided. In this optical signal generation circuit, an inputted optical NRZ signal is split in two by an optical coupler, and the first half is presented to a clock extraction circuit. The second half is presented to an EA modulator. The clock extraction circuit extracts the clock component from the optical NRZ signal and presents the result to the EA modulator. The EA modulator is energized by the clock component received, and thus generates an optical RZ signal as an output. In this case, a delay circuit should preferably be provided to the preceding stage of the EA modulator to adjust the timing. It is also preferable for the output of the EA modulator to be fed back to the delay circuit to control the delay amount.
The optical signal generation circuit according to a second aspect of the present invention has a first waveguide and a second waveguide. Parts of the two waveguides are placed close to each other, forming a coupled waveguide region. An optical NRZ signal and an RZ pulse train have the same wavelengths and are inputted to the first waveguide. At this point, a nonlinear phase shift based on cross-phase modulation is induced in the RZ pulse train by the optical NRZ signal in the coupled region. As a result, only the RZ pulses corresponding to the optical NRZ signal (from among the individual RZ pulses constituting the RZ pulse train) are outputted by the first waveguide.
At this time, an optical NRZ signal leaked out from the output terminal of the first waveguide can be removed if the optical NRZ signal and the RZ pulse train have different wavelengths and the output terminal of the first waveguide is provided with an optical band-pass filter. It is also possible to provide the output terminal of the second waveguide with a receiver to receive RZ pulses that do not correspond to the optical NRZ signal and to monitor the operation of the optical signal generation circuit.
The optical signal generation circuit according to a third aspect of the present invention has a Y-branch light waveguide. The primary mode and first mode of signal light propagate through this Y-branch light waveguide. Thus, signal light appears only in one of these Y-branches due to the interference between these two modes if the length from the input point of signal light to the Y-branch point is appropriately set. The Y-branch light waveguide receives an optical NRZ signal having a prescribed wavelength, and an RZ pulse train whose wavelength is different from this prescribed wavelength
At this point, a nonlinear phase shift based on cross-phase modulation is induced in the RZ pulse train by the optical NRZ signal in the Y-branch light waveguide. As a result, only the RZ pulses corresponding to the optical NRZ signal (from among the individual RZ pulses constituting the RZ pulse train) appear in one of the Y-branches. The RZ pulses that do not correspond to the optical NRZ signal appear in the other Y-branch. In this embodiment as well, it is beneficial to provide the output portion with an optical band-pass filter or a receiver for receiving the RZ pulses that do not correspond to the optical NRZ signal.
The optical signal generation circuit according to a fourth aspect of the present invention has a light waveguide and a polarizer connected to this light waveguide. The light waveguide is presented with an optical NRZ signal having a first polarization direction and an RZ pulse train having a second polarization direction, which differs by 45xc2x0 from the first polarization direction. As a result, a nonlinear phase shift based on cross-phase modulation is induced in the RZ pulse train by the optical NRZ signal in the light waveguide. As a result, the polarization plane at the outgoing end of an optical fiber 51 for the RZ pulse train can be shifted relative to the initial polarization plane. The RZ pulse train is therefore inputted to the polarizer, and only the RZ pulses corresponding to the optical NRZ signal are extracted.
The optical transmission line according to a fifth embodiment of the present invention has first and second dispersion compensation means, which are provided to the preceding and subsequent stages, respectively, of an optical fiber transmission line for transmitting optical pulses. Here, the dispersion value of the first dispersion compensation means is set substantially to near-zero level or to a normal dispersion level. The dispersion value of the second dispersion compensation means is negative. Pulse widening in the optical fiber transmission line is therefore controlled by the nonlinear chirp induced in the first dispersion compensation means.